1. Field of the Invention
The present invention relates to a fuser and an image forming apparatus, and more particularly to a fuser having a fusing belt and an image forming apparatus including the fuser.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatuses includes a fuser for thermally fixing a toner image transferred to a printed medium to the printed medium. A fuser of this kind includes an endless fusing belt configured to transfer heat to a toner image, a pressure roller configured to hold the printed medium having the toner image transferred thereon, between the pressure roller and the fusing belt, and a heat source configured to heat the fusing belt. In addition, in order to prevent ignition caused by abnormal overheating of the fusing belt, the fuser also includes a detection unit provided on the outer peripheral side of the fusing belt, and detect abnormal overheating of the fusing belt by monitoring the temperature of the fusing belt, and a mechanism for cutting off power supplied to the heat source based on the temperature of the fusing belt detected by the detection unit. As one of such fusers, a fuser described in JP 2009-276570 A (hereinafter, referred to as conventional fuser) is known.
When the temperature of the fusing belt is raised by abnormal heating of the heat source, the fusing belt tends to expand at an initial stage, and finally ignite through contraction. Upon contraction of the fusing belt, the fusing belt and the detection unit provided on the outer peripheral side are separated in distance, so that measurement of the temperature by the detection unit is made difficult, and detection of the abnormal overheating of the fusing belt is delayed. In order to prevent such a situation, the conventional fuser is provided with a guide member near the detection unit and on the inner peripheral side of the fusing belt to prevent separation in distance between the fusing belt and the detection unit. However, since the conventional fuser is provided with the guide member, heat from the heat source reaches the fusing belt through the guide member, and rise in temperature of the fusing belt near the guide member is delayed relative to temperature rise at the other part of the fusing belt. Accordingly, the conventional fuser has a problem that the delayed detection of the abnormal overheating of the fusing belt by the detection unit cannot prevent the ignition of the fusing belt.